1. Field of the Invention
This invention relates to dual screw-type motors and pumps in general, and, in particular, to both dry and lubricated vacuum pumps, pneumatic and air-conditioning compressors, hydraulic pumps, pneumatic motors, and hydraulic motors.
2. Description of Related Art
The prior art includes a number of efforts to produce effective screw-type rotor pumps and motors. In some rare cases, the rotor is tapered and the flights of the screw portion are graduated so as to be wider at one end.
Perhaps most typical of these special cases is the pump described in U.S. Pat. No. 6,672,855 issued to Michael Henry North on Jan. 6, 2004 and illustrated in FIG. 1 as Prior Art. In this design the inner-cone diameter, referred to as the “root diameter”, increases as the screw moves from the inlet side to the outlet side. This design appears to allow for improved volume compression characteristics. The varying pitched threads also allow for greater pumping speed at relatively “low” (under 50 mbar) inlet pressures as well.
A similar earlier effort is described in U.S. Pat. No. 6,019,586 which discloses a screw compressor/pump including a graduated, contracted screw portion. The patent refers to “an inner cone tapered towards a suction port side”, and a rotor chamber “defining an outer cone tapered toward the discharge port side, thus forming a gradationally contracted cavity between the spiral groove and the conical wall surface of the rotor chambers.” The result is a pump/compressor which provides for volume compression as well as a shortening of the rotor and shaft. This design appears to improve pump sealing characteristics somewhat, but the patent reference only states that “the helix tooth has its top land surface approximating very closely to the inside wall of the rotor chamber to minimize the clearance and gas leakage.”
Of possible lesser relevance is the device described in U.S. Publication US 2001/0041145 A1, which discloses a vacuum pump including a body, a pumping chamber, and tapered rotors.
The general state of the art can also be found in the following: U.S. Pat. Nos. 4,952,125; 6,129,534; 6,217,305; and, 6,379,135. Note also the following U.S. Publications: 2001/0024620 A1; 2001/0041145 A1; and, 2001/0051101 A1. In addition note the following patent applications or references: Japanese Patent/Application 2001/304,156; 2000/073,976; and, 01267384. Also European application number 1130263 A1 and French patent number 2684417A1.
There tends to be the following shortcoming with regard to the foregoing prior art for screw-type pumps and motors:
First, they often have poor sealing characteristics, resulting in excessive energy consumption and lower pumping efficiency. This especially becomes a problem over time, because the gaps between enmeshed screws become larger as friction wears the screws against each other.
Second, they tend to have large rotor size and shaft sizes compared to their pumping capacities. Pump/motor size to output ratio is thus an issue.
Third, while some of the cited tapered screw pumps have limited volume compression, standard screw pumps have no volume compression along the rotor length. This makes it difficult to use such pumps/motors for gas applications, and also requires more power consumption than a pump which achieves compression along the rotor.
Fourth, such pumps/motors tend to have limited pressure differential for low viscosity flows.
The device described in U.S. Pat. No. 6,672,855 and some others attempt to address some of these issues, and while such inventions are useful, their versions still suffer from other significant drawbacks:
First, the pump designs do not relieve pressures within the pump that exceeds the high-side pressure. At the start of pump operations, when both sides are at the same pressure, the pump internally may develop pressures higher than the “high pressure” side. This causes a large loss of efficiency, and diminishes the seal which slows down pumping action. U.S. Pat. No. 6,672,855 suggests the use of an electronic regulator to reduce shaft speed at the initial stages of pump operation to minimize the problem, but at the cost of slower pumping speed.
Second, the outer cone, i.e. “thread diameter” taper described in U.S. Pat. Nos. 6,672,855 and 6,019,586 both work against optimal sealing efficiency. As the pumps operate, the pressure differential from the high side to the low side will naturally tend to push the screws out of the block, i.e. the rotor chamber. With respect to vacuum pumps, at the start of the pumping cycle, the intra-cavity pressure is higher than the output pressure (atmospheric pressure); thus the problem is exacerbated. The inventions described in U.S. Pat. Nos. 6,672,855 and 6,019,586 both include a truncated cone with the wider diameter at the inlet, i.e. low, pressure side. In a two dimensional view, the block is a trapezoid with the longer length at the inlet side. The outlet, i.e. high, pressure side tends to force the rotors out of the block, which reduces sealing characteristics between the block edge and the threads.
While both such prior art devices can adequately handle the pressure differential required for vacuum pumps (about 14.7 psi or 760 mmHg), both are unfit for pumping across much larger pressure differentials. In gas compressors, for instance, the pressure differential could be as large as 5,000 psi.